1. Field of the Invention
The present invention relates to an image signal processing method and unit for performing nonlinear compensation on image signals, and to an image display device using the method and unit.
2. Description of the Related Art
Human visual characteristics are nonlinear. In particular, the degree of brightness human beings feel has characteristics of a logarithmic function with respect to luminance, as shown in FIG. 5. In a cathode ray tube (CRT), the display luminance with respect to an image signal has so-called gamma characteristics, as shown in FIG. 6A. Thus, in a display device using CRTs, the gamma characteristics and visual characteristics of a logarithmic function cancel out each other and the degree of brightness human beings feel has characteristics substantially linear with respect to image signals.
In contrast, newly-developed display devices, such as a digital mirror device (DMD) and plasma display panel (PDP), do not have the gamma characteristics. As shown in FIG. 6B, these display devices have characteristics in which the display luminance is linear with respect to image signals. Therefore, in these display devices, the degree of brightness human beings feel exhibits characteristics of a logarithmic function with respect to image signals. For this reason, to compensate the degree of brightness of image signals human beings feel to have linear characteristics, nonlinear compensation must be performed on the image signals. Proposed as one of the methods is using a look-up table (hereinafter abbreviated as “LUT”) to perform nonlinear compensation, as shown in FIG. 7 (see Japanese Patent Unexamined Publication H10-153983, for example).
FIG. 8 is a table showing an example of input-output characteristics in such a LUT. In this table, “input gradation” is an image signal before compensation, and the signal levels of the image signals to be fed into the LUT are expressed in 256 steps, i.e. “0” to “255”. “Display gradation” is an image signal after compensation, and the signal levels supplied from the LUT are standardized so that the maximum value thereof is “255”. In FIG. 8, the display gradations are shown only when the input gradations are “0”, “10”, “20”, and so on, to “250”.
When a display device is installed in a dark place, and no ambient light incident on the display device, nonlinear compensation is performed on the input gradations to be fed into a display device having no gamma characteristics, such as a PDP, as shown in FIG. 8. Then, the relation between the input gradation and display luminance exhibits gamma characteristics, as shown by characteristic curve A in FIG. 9A. Thus, the degree of brightness human beings feel and input gradations are substantially linear, as shown by characteristics curve B in FIG. 9B.
However, when the display device is installed in a bright place, the display luminance is uniformly increased by the influence of ambient light, as shown by characteristic curve C in FIG. 9A. Thus, even the conventional nonlinear compensation cannot return the degree of brightness human beings feel so as to have linear characteristics. Characteristic curve D in FIG. 9B shows the degree of brightness human beings feel with respect to the input gradations at that time. Human beings cannot recognize a change in gradation clearly in the portion of low input gradations.
This phenomenon depends on the human visual characteristics and can be considered as follows. Assume that the display gradations shown in FIG. 8 indicate the display luminance in a dark place. Then, when the input gradation increases from 10 gradations to 20 gradations, the display luminance increases “0.2” to “0.9”, thus making human beings feel that the brightness is 4.5 times. In contrast, assume the display luminance increases by a luminance corresponding to “30”, for example, in a bright place, under the influence of ambient light incident to the display device. In this case, even when the input gradation increases from 10 gradations to 20 gradations, the display luminance increases from “30.2” to “30.9” only. Thus, human beings almost cannot feel a change in brightness. Of course, for a brighter image, the ratio of the display luminance between 240 gradations and 250 gradations in a dark place, for example, is 244.1/223.2=1.09. The ratio between the same gradations in a bright place is 274.1/253.2=1.08. These figures do not have a large difference and thus the influence of ambient light is negligible.
As described above, for the conventional method of nonlinear compensation, because a display device is influenced by the ambient light incident thereon, the degree of brightness human beings feel cannot be compensated to have linear characteristics. Particularly in a dark image having low input gradations, a change in gradation cannot be recognized and the gradations become uniform.
The present invention addresses this problem and aims to provide an image signal processing method and unit capable of compensating the characteristics of brightness of image signals human beings feel, even when the display luminance of a display device has characteristics linear with respect to the image signals and the display luminance is uniformly increased by the influence of ambient light incident to the display device. The present invention also aims to provide an image display using the method and unit.